Fraunhofer line

In physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer (1787--1826). The lines were originally observed as dark features in the optical spectrum of the Sun.

The English chemist William Hyde Wollaston was in 1802 the first person to note the appearance of a number of dark features in the solar spectrum. In 1814, Fraunhofer independently rediscovered the lines and began a systematic study and careful measurement of the wavelength of these features. In all, he mapped over 570 lines, and designated the principle features with the letters A through K, and weaker lines with other letters.

It was later discovered by Kirchoff and Bunsen that each chemical element was associated with a set of spectral lines, and deduced that the dark lines in the solar spectrum were caused by absorption by those elements in the upper layers of the sun. Some of the observed features are also caused by absorption in Oxygen molecules in the atmosphere.

The major Fraunhofer lines, and the elements they are associated with, are shown in the following table:

Designation Element Wavelength (nm) Designation Element Wavelength (nm)
y O₂ 898.765 c Fe 495.761
Z O₂ 822.696 F H β 486.134
A O₂ 759.370 d Fe 466.814
B O₂ 686.719 e Fe 438.355
C H α 656.281 G' H γ 434.047
a O₂ 627.661 G Fe 430.790
D₁ Na 589.594 G Ca 430.774
D₂ Na 588.997 h H δ 410.175
D₃ He 587.565 H Ca⁺ 396.847
E₂ Fe 527.039 K Ca⁺ 393.368
b₁ Mg 518.362 L Fe 382.044
b₂ Mg 517.270 N Fe 358.121
b₃ Fe 516.891 P Ti⁺ 336.112
b₄ Fe 516.751 T Fe 302.108
b₄ Mg 516.733 t Ni 299.444

The Fraunhofer C-, F-, G'-, and h- lines correspond to the alpha, beta, gamma and delta lines of the Balmer series of emission lines of the hydrogen atom.

Because of their well defined wavelengths, Fraunhofer lines are often used to characterize the refractive index and dispersion properties of optical materials.